update help

prettyjson.dart

@@ -5,7 +5,7 @@ import 'package:args/args.dart';
 main(List<String> args) async {
   final parseResult = parseArgs(args);
   if (parseResult['help']) {
-    print('Print help message for prettyjson.dart');
+    printHelp();
     return;
   }
   final restArgs = parseResult.rest;
@@ -39,3 +39,12 @@ Future<String> readFromStdin() async {
     }
     return Future<String>.value(stdinContentArray.join('\n'));
 }
+
+printHelp() {
+  print('pretty.dart - version 0.1');
+  print('Usage:');
+  print('  prettyjson.dart [OPTIONS] [FILE NAME]');
+  print('');
+  print('Optional arguments:');
+  print('  -h,--help             Show this help message and exit');
+}